Television receiving system



Jan. 7, 1941. C. w. CARNAHAN 2,227,630

TELEVISION RECEIVING SYSTEM Filed Nov. 5, 1937 2 Sheets- Sheet 1 SIGMA L A MPLITl/DE 3 laid:

INVENTOR Jan. 7, 1941. w A N N 2,227,630

TELEVISION RECEIVING SYSTEM Filed NOV. 5, 1937 2 Sheets-Sheet 2 Patented Jan. 7, 1941 UNITED STATES TELEVISION RECEIVING SYSTEM Chalon W. Carnahan, St. Marys, Pa., ass'ignor to Hygrade Sylvania Corporation, Salem, Mass,

a corporation of Massachusetts Application November 5, 1937, Serial No. 172,920

16 Claims.

This invention relates to eleotro-optical systems and more especially to methods and means for electro-optically transmitting and receiving visual representations at a distance including amongst others such systems as television, picture transmission, telephoto and facsimile systems generally.

A principal object of the invention relates to a novel method of increasing the brightness contrast at the boundary between adjacent light and dark areas of a reproduced visual representation.

Another principal object is to provide an improved method of compensating locally at a television receiverrthe brightness distortion introduced by the frequency limitations of the transmitting link between a source of television currents and the television reproducing device.

Another object is to improve the delineation of a picture or image reproduced on a cathoderay tube screen, particularly between adjacent areas exhibiting sudden or pronounced brightness contrast.

Heretofore, there have beenknown two generic types of systems for transmitting picture or image currents into a b'rilliancy-modulated fluorescent spot. One system referred to herein as intensity modulation, employs for scanning, a cathode-ray beam of uniform scanning velocity the intensity of the beam being varied to control the brightness of the fluorescent spot. In the other system referred to as velocity modulation, the cathode-ray scanning beam is of uniform intensity but has its instantaneous velocity varied to vary correspondingly the brightness of the fluorescent spot. In the intensity modulation system the brightness modulations are usually effected by modulating the voltage of a modulating electrode adjacent the electron-emitter of a cathode-ray tube. modulating electrode acts on the cathode beam current in the same manner as the control grid of a radio tube acts on the plate current thereof. The scanning movement in connection with intensity modulation is carried out by diverting the cathode-ray stant, and the brightness modulation at the finerescent screen is obtained by varying the scanning speed of the beam. Consequently, the brightness varies inversely with the speed of the beam over the fluorescent screen. 5

It is another important object of the present invention therefore to provide a method and apparatus for scanning a fluorescent screen by a cathode-ray beam, wherein the brilliancy of the fluorescent spot can be controlled by intensity modulation as well as by velocity modulation.

A feature of the invention relates to a rnethocv and means for imparting extreme and sudden brightness to predetermined parts of a visual representation reproduced under control of a cathode-ray beam, by supplementing the intensity modulation of the beam by a local velocity modulation thereof.

Another feature relates to the method and means to compensate for blurring of the boundary between adjacent contrasting areas result ing from the frequency band limitations of the network between the image current source and the image reproducer.

Another feature relates to a method and means for supplementing the usual intensity modulation by a velocity modulation only when the received modulated signal curve exhibits a slope exceeding a predetermined value either positive or negative. 0

Another feature relates to the means for introducing a velocity modulated signal which is substantially independent of the absolute value of the rate of change of brightness beyond a predetermined limit.

Another feature relates to the means for reducing the intensity of a cathode-ray scanning beam during the time a velocity modulation is being imparted thereto.

A still further feature relates to the novel organization, arrangement and interconnection of parts whereby improved delineation between contrasting areas of a reproduced television image is achieved. 45

Other features and advantages not specifically enumerated will be apparent after a consideration of the following detailed description and the appended claims.

While the invention will be disclosed herein as embodied in certain specific circuits and apparatus, it will be understood that the invention is not necessarily limited thereto and that in certain of its phases, the inventive concept is applicable to other types of signal communicating systems. Accordingly in the drawings,

Fig. 1 shows a characteristic curve to explain the theory and operation of the invention.

Fig. 2 is a detailed schematic composite circuit diagram of a typical system embodying the invention.

Figs. 3 to 3 show a series of curves also explanatory of the invention.

Fig. 4 is a generalized schematic representation of a typical system embodying the invention.

Referring more particularly to Fig. l, the curve A represents the modulation curve of a voltage or current signal at the input end of a frequency limited transmission network or device. The abscissae represent the duration of the signal and the ordinates represent the instantaneous magni. tude thereof. Because of the frequency-band limitations of the transmitting network or device,

the shape of the output signal corresponding to the input signal of curve A is modified qualitatively to a curve of the general shape of curve B. In the case of television transmission where the modulating frequencies are of a relatively high order, the frequency limitations of the transmitting device or transmitting network become of considerable importance. For example, this frequency limitation may be determined by the frequency characteristics of the amplifiers in the transmitter and receiver, and by the permissible width of the side bands during transmission. The result of such a frequency limitation is that a signal variation corresponding to a pronounced brightness contrast at the transmitting end, for example that represented by a rectangular signal front (curve A) is modified to the general shape of curve B when the signal is received at the modulating grid of the receiving cathode-ray tube or the like.

For purposes of explanation the network ahead of the reproducing device may be considered as a low-pass filter with uniform frequency characteristics from zero frequency up to a certain maximum or cut-off frequency, fmax. Actually, a band-pass filter is to be considered, With minor restrictions the effective half-width of the passed frequency band takes the place of the fmax. It is known in the art that the slope at the midpoint of the received signal is proportional to the frequency fmax and to the increase in amplitude of the input signal, The duration of the transition, At, in Fig. 1 is the same for all rectangular input signal variations, independent of the absolute value, Ah, of the variations. As the signals are applied tomodulate the tone of brightness at the fluorescent screen, the rate of change in brightness between adjacent elemental areas will be referred to herein as the tone transition.

In order therefore that the tone transitions may be reproduced as faithfully as possible by employing a supplementary velocity modulation, the following conditions should be fulfilled:

1. The velocity modulation signal should have a duration less than, or equal to, the duration of the tone transition, and the direction of modulation should be independent of the direction of the change of tone, i. e., independent of whether the transition is from a lighter to a darker tone or vice versa.

2. The velocity modulation signal should have a more or less constant amplitude independent of the magnitude of the tone transition.

3. The intensity of the scanning spot should be reduced for the duration of the velocity modulation to overcome the apparent increase in intensity due to the stationary spot.

4. The velocity modulation signal should be derived from the intensity modulation in such a manner that only sharp tone transitions appear in the derived signal.

5. Since, if condition 4 is met, high frequency components will be overemphasized, means should be provided for selecting only those components in the derived signal which are greater in mag- .nitude than the high frequency interference components.

Referring to Figs. 2 and the curves of Figs. 3' to 3 a description will now be given of a circuit arrangement which will fulfill the above conditions. The numeral |4 represents the network preceding the image reproducing equipment and may be any well known form of image current transmitter or it may represent the incoming terminal equipment of an image current receiver. The received image currents are fed through a suitable coupling condenser |5 to the control grid I8 of a suitable amplifier tube [1, preferably, although not necessarily, of the screen-grid type having the electron-emitting cathode l8, controlgrid l6, screen-grid l9, and anode or plate 20. Plate 20 is supplied with the necessary D, C. operating potential from a suitable positive terminal of a conventional B power supply arrangement, through resistor 2|, The amplified signal variations across resistor 2| are applied through cou pling condenser 22 to the modulating electrode or control grid 5 of the cathode-ray tube 2. It will be understood of course that the grid 5 and cathode 4 of the cathode-raytube are suitably biased and provide a return path through ground to the negative terminal of the B power supply circuit which is connected to the resistor 2|. The tube I1 is provided with the usual grid leak resistor 23, and control grid bias resistor 24 with its bypass condenser 25.

The arriving image current signal is also fed through coupling condenser 26 to the control grid 27 of a screen-grid amplifier tube 28 similar to tube I1, The plate 29 of this tube is connected to a suitable positive terminal 30 of the same B power supply as above described, through an inductance 3|. The plates or anodes 32, 33 of a full wave rectifier tube 34 are connected across the terminals of inductance 3|, the electrical midpoint of which is returned through resistor 35 to the electron-emitting cathode 36. 1

A signal impressed upon control grid 21 of the shape shown in either curve C or D of Fig. 3 will therefore cause a positive voltage surge of the shape corresponding to the derivative thereof as represented in Fig. 3 to be impressed upon control grid 38. The terminal 40 of the grid leak resistor 4| is over-biased negatively by means of a suitable biasing arrangement 42 to such on extent that signals with a first derivative below a predetermined limit h (Fig. 3*), will not cause any current to flow in the plate circuit of tube 39. Tube 39 therefore is the threshold discriminator device which fulfills condition 4 supra. Tubes 28 and 34 together with inductance 3| fulfill condition 1 supra.

The signal passed by tube 39 results in a voltage drop across resistor 43 and is fed through coupling condenser 44 to the control grid 45 of screen-grid tube 46, and the impressed signal on said grid 45 is of the general shape shown in Fig. 3. The control grid 45 is suitably biased negacontrol grid of tube 46.

tively by the resistor 46 and it's b'y-pass condenser 41, and the said grid is returned to the cathode 48 through the high resistance potentiometer 49. In the plate circuit of tube 46 the signal is again positive as shown in Fig. 3 and causes an accumulation of a positive charge on condenser 56 because of the current fiowthrough the diode rectifier tube 5!, the current fiowing when the plate 52 of the tube 5| becomes sulficiently positive due to the negative signal at the The voltage across condenser 56, represented by Fig. 3 is fed through coupling condenser 53 to the control grid 54 of a screen grid tube 55 provided with-the grid leak resistor 56 and the grid -biasing arrangement 51, 58. The increments in the plate current of tube 55 are therefore applied to the scanning delay elements 8 to modify the line scanning velocity controlled primarily by the linescanning elements I. With reference to tube 46 thecontrol grid thereof is biased sufficiently negative and the tube has such a high mutual conductance that any signal passed by the threshold tube 39 causes the tube 46 to operate in the plate current cut-off region of its characteristic curve. Thus tube 46 fulfills condition 2 supra. Fulfillment of condition 3is attained by tapping off a voltage from the potentiometer 49 by means of conductor 59 which applies a negative impulse to the control grid 66 of the buffer tube 6!, the cathode 62 of which is connected to ground through resistor 63. The voltages developed across resistor 63 are fed through coupling condensers 64 and 22 to the modulator grid 5, thus reducing the beam current simultaneously with the application of the velocity modulating impulse to the coils 8 as above described. Fulfillment of condition 5 can be attained by the proper choice of the bias voltage on the control grid of the threshold tube 39. An additional tube 65 is provided, the control grid of which is fed with a positive impulse from the usual line scanning voltage generator 66 at the end of each scanned line. This tube 65 is rendered conductive at the end of each scanned line to wipe out any residual charge that may exist on condenser 56 as a result of the retouching or compensating impulses described above. In addition to, or in place'of, tube 65 there may be provided a resistor 61 to discharge condenser 56 slowly if desired.

Fig. 4 shows a generalized system described above. In this figure, the numeral I represents a I network connecting a source of image currents I scanning pattern under control of the deflection elements 1. Associated with the defiection elel ments 1 are similar deflection elements 8 which are so arranged with respect to elements 1, that when elements 8 are energized they tend to modify the velocity of the scanning beam.

- The received signal is passed to the modulating amplifier 9 which may be of any well known type, and'partof the output of said amplifier is applied across the modulating electrode 5 and the cathode 4 in the usual manner. A portion of the received signal is also fed through the derivative operator l6 by means of which the time derivative of the original signal is obtained. The

output of the network of the negative impulse from the output of device H is also applied to the modulating amplifier 9 thereby reducing the-beam intensity for the duration of the delay impulse applied to deflection elements 8. Consequently, should the transition between certain adjacent areas of the image be represented by a received signal having a slope above a predetermined value, thus indicating sudden brightness transition in the original, the boundaries between said areas in the reproduced image are velocity modulated so as to impart a corresponding sudden brightness change thereto over and above that which would be normally reproduced Without the velocity modulation compensation.

Divers changes and modifications may be made without departing from the spirit and scope of the invention. For example it is preferred to arrange the values of the circuit elements set forth so that the time of action of the auxiliary or delay scanning field is not of longer duration than the actual duration of the tone transition to be retouched. On the other hand the circuit elements may-be of such values that while the delay field may operate over a time interval slightly longer than the actual duration of the tone transition to be retouched, insofar as its physiological effect on the eye is concerned, it is substantially equal to the actual duration of the tone transition to be retouched.

After having described the nature of the invention in detail, it may be advisable to summarize the essentials and to say definitely what" is believed to be an improvement over the known prior art.

Because of the unavoidable frequency limitation. of electric circuits, any television picture transmitted electrically loses contrast. The steepest wave front that can be transmitted, viz. a vertical front, is softened. The response at the receiving end for a vertical front wave is represented in Fig. 1 by the curve B. Any wave, front less steep than vertical at the transmission" end arrives at the receiving end with a shape which is less steep than curve B in Fig. 1.

According to the invention, whenever a wave front is received which is as steep as the one corresponding to a vertical transmitted front, or" nearly as steep, a threshold device picks that wave front out and operates a supplementary modulation device, the function of which is to increase the local brightness contrast. As the contrast is proportional to the first derivative of the signal itself, the selective action on such wave fronts is controlled by a threshold device sensitive to the first derivative of the signal.

What I claim is:

1. In an image reproducing system wherein the reproduction is effected mainly by a constant velocity scanning beam and mainly by intensity modulations, the method which includes the steps of developing a fluorescent scanning spot under control of a cathoderay beam, moving said spot in a predetermined scanning pattern at a substantially uniform velocity, modulating said spot mainly by intensity-modulation signals, temporarily modifying said velocity only when the rate of change of the tone transition is above a pre determined value, the temporary velocity-modulations being all of substantially the same magnitude and substantially independent of the instantaneous rate of the tone transition so long as the said rate is above said predetermined value during a predetermined short time interval.

2. The method of image reproduction wherein the image is reproduced mainly by intensitymodulations of a fluorescent spot, which includes the steps of developing a fluorescent spot under control of a cathode-ray beam, developing a main intensity-modulating impulse in accord-ance with the tone of an elementary area of the image, developing a supplemental velocity-modulating impulse under control of the first derivative of said intensity-modulating impulse, controlling the intensity of the fluorescent spot by both said impulses to compensate for frequency distortion in a coupling network ahead of the reproducer, developing a third impulse to counteract the intensity-modulating impulse to a predetermined extent, all of said impulses being applied simultaneously to control the apparent intensity of the fluorescent spot.

3. In an image reproducing system wherein the image is reproduced mainly by intensity modulations of a cathode-ray beam, means to develop a fluorescent spot under control of a cathode-ray beam, means to develop an intensity-modulating impulse in accordance with the tone of an elementary area of the image, means to develop a scanning velocity-modulating impulse under control of the first derivative of said intensity-modulating impulse, means to control the intensity of the fluorescent spot by both said impulses to compensate for frequency distortion in a coupling network ahead of the reproducer, means to derive a third impulse to counteract the intensitymodulating impulse of predetermined extent, and means to apply the three impulses simultaneously to control the apparent intensity of the fluorescent spot.

4. In a television reproducer of the cathoderay tube type wherein .the image is reproduced mainly by intensity modulations of a cathoderay beam, means to reproduce an image on the fluorescent screen of the tube primarily by intensity modulation of the cathode-ray beam, and means to retouch the intensity of the fluorescent spot for lack of high lights including means to derive from the received image signal an impulse which corresponds to the first derivative thereof, scanning delay means to subject the scanning beam to a delay field, and means including a threshold discriminator tube to cause current to flow to said delay means only when signals are received whose first derivatives are above a predetermined value.

5. A system according to claim 4 in which the last-mentioned means includes a negatively overbiased electron tube.

6. A system according to claim 4 in which the means to derive said impulse corresponding to the first derivative of the received signal includes an electron tube upon the control grid of which the received signal is impressed and an inductance in the plate circuit of said tube, together with a full-wave rectifier connected across the ends of said inductance.

'7. A system according to claim 4 in which an electrontube is' provided for reversing and limiting the amplitude o'f-the signal flowing to the scanning delay means said tube having such a characteristic that its plate currentis out 01f by any signal which is passed by the threshold discriminator tube.

8. In an image reproducing system wherein the image is reproduced mainly by intensitymodulation of a cathode-ray beam the combination of a cathode-ray tube image reproducer having a fluorescent screen with means to develop and focus a beam of electrons on said screen and with means to develop a scanning field to deflect the beam in successive scanning lines over said screen, auxiliary field producing means to modulate the velocity of the beam during its scanning movement, an intensity modulating electrode for said tube, means to apply modulating potentials to said electrode under control of received image signals, a condenser for receiving a charge under control of the first derivatives of said received signals when said derivatives are above a predetermined value, means to energize said auxiliary field producing means under control of said charge, and means to discharge said condenser at the end of each scanned line. 7

9. A system according to claim 8 in which the means for discharging said condenser includes an electron tube which is connected to the condenser and is rendered conductive at the end of each line scanning.

10. A system according to claim 8 in which said condenser is charged under control of a derivative developing network, a threshold discriminating network, and an amplitude limiting network so that said auxiliary field is energized only when a sudden brightness is to be imparted to the image corresponding to sudden tone transition in the original.

11. In an image reproducing system, the combination of a cathode-ray tube image reproducer having a fluorescent screen with means to develop and focus a beam of electrons on said screen and with means to develop a scanning field to deflect the beam in successive scanning lines over said screen, auxiliary field producing means to modulate the velocity of the beam during its scanning movement, an intensity modulating electrode for said beam, means to apply intensity modulating impulses to said electrode under control of received image signals, and means to apply velocity modulating impulses to said auxiliary field producing means only when sudden tone transitions are to be reproduced, the

last-mentioned means including a network upon which a portion of the signal currents is impressed to derive a signal corresponding to the first derivative of the received signal, a threshold discriminator network connected to the foregoing network to pass a signal only when the first derivative is above a predetermined value, an amplitude limiting network connected to the discriminator network for producing a signal of substantially uniform amplitude from that passed by the discriminator, a condenser, means to charge said condenser under control of the signal passed by said limiting network, and means to energize said auxiliary field producing means under control of the charge on said corfdenser.

12. A system according to claim 11 in which the network for deriving the first derivative signal includes an electron tube having an inductance in the output circuit thereof and a full- 15. A system according to claim 11 in which means are provided for applying to said intensity modulating electrode a signal derived from the threshold discriminator network, said signal acting to reduce the intensity of the beam only while 5 said auxiliary field producing means is being energized by said velocity modulating impulses.

16. A system according to claim 11 in which an additional network is provided to discharge said condenser at the end of each line scanning. 10

CHALON w. CARNAHAN. 

